Add dark mode to nicegui.io

[suspiciousRaccoon]

Unless I've missed it the website doesn't have a button or option to switch between light and dark mode. Adding this would be nice for coding during the night, or vampires who hate light 🧛.
I'm not sure how dark mode is implemented in NiceGUI but it might conflict with the dark mode demo in the docs.

[falkoschindler]

Good point, @suspiciousRaccoon!
In fact, there's already a feature request: #771.
Therefore I'd suggest to continue the discussion over there. 🙂